Nanowire based LEDs (Light Emitting Diodes) are comprised of semiconductor nanowires or arrays of semiconductor nanowires grown on a substrate, like for example silicon or GaN. Typically on such substrate a planar buffer layer is grown first and subsequently an array of nanowires is grown on the surface of the buffer layer. The buffer layer is used as the base layer for growing the nanowires. Additionally, it can serve for electrical current transport. The buffer layer is usually transparent for the light emitted by the LED.
Each nanowire protrudes from the buffer layer and contains multiple regions of materials forming p-i-njunctions around the nanowire core or along the nanowire axis. When charge carriers are injected into the respective p- and n-regions, they recombine in the i-region, and this recombination generates light. The light is generated inside each nanowire randomly and emitted in all directions. One problem with such a structure is that a substantial fraction of the generated light is wasted, as only a portion is directed in a desired direction.
Another problem associated with nanowire based LEDs is that this structure relies on the conductivity of the buffer layer for current transport into the active region, the p-i-n-junction. For large devices the distance between the contact and the nanowires within the LED can be considerable, causing voltage drop and resistive losses over the buffer layer. Carrier recombination and light generation will happen predominantly near the contact pad on the n-contact side causing current crowding and non-uniform luminance. This problem remains when mounting the LED device onto a carrier supplying the LED device with current for light generation.
The difference between a pn-junction and a p-i-n-junction is that the latter has a wider active region. The wider active region allows for a higher probability of recombination in the i-region, thus generation of light, although both pn- and p-i-n-junctions can be used for light generation in LED devices.